The present invention relates to a tuning circuit for high frequency receivers, particularly of the type employing a digital input keyboard for reading in of the desired tuning frequency and electronic tuning adjustment. High frequency receivers are generally tuned by rotating a tuning knob to sweep a frequency band which has been preselected by means of a frequency band switch. Indication of the current tuning location is provided by an indicator which moves along a scale. With such an analog type of tuning, the indication of the selected station on the scale is relatively inaccurate due to the large magnitude of the associated mechanical tolerances, so that the identity of the station to which the set is tuned cannot be ascertained with certainty from the location of the indicator on the scale. Moreover, it is necessary, in order to tune for perfect reception of the respective station, to accurately tune to a optimum setting, which usually requires an additional fine tuning indicator.
It is also known, as disclosed in the periodical "Funkschau", 1974, at pages 62 to 63 and 93 to 95, to tune VHF receivers by means of a digital input keyboard and digital frequency indication, if the receiver is provided with variable capacitance diodes. A counting device is provided to periodically count the oscillations of the mixing oscillator, under consideration of the intermediate frequency, so that the counter state displayed on an optical indicator device indicates the current receiving frequency.
For tuning to a desired station, the receiving frequency assigned to the station is punched into the keyboard, which has a series-connected coder, and is fed to a comparator. The counter state of the above-mentioned counting device is compared, by digits, with a number representing the receiving frequency, which has been keyed into a shift register of the comparator. The results of the comparisons serve to produce a control voltage for the search operation.
In the known circuit, however, there exists the drawback that, due to the digit oriented comparison, it is necessary to provide a number of comparators equal to the maximum available number of digits. In the above-mentioned periodical these comparators are called 7485 comparators. It is further necessary, in order to be able to supply the individual comparators with the numbers to be compared, to provide two additional memories for each comparator.
A further drawback is that during the course of data input, comparison is being effected between digits which are not of the same order, or significance, and this initially produces the wrong tuning voltage. This is so because, for example, the first digit fed in through the input keyboard, which is normally the highest-order, or most significant, digit, passes sucessively through those memories which are connected ahead of the memory actually assigned to that digit order. Only after all digits have been fed in will the digits of the corresponding orders of the two numbers being compared be compared correctly.
It can happen, for example, that during writing in of the desired receiving frequency value, the most significant digit of the written-in number is compared with the least significant digit of the number furnished by the counting device and this will initially produce the wrong tuning voltage. In the known circuit, special measures are required, moreover, with respect to obtaining the optimum comparison such that during comparison the individual digits are subject to different weightings and a deviation between the highest order digits causes the resulting tuning voltage to be higher than when there is a deviation between lower order digits.